In many testing apparatuses, such as a coagulation instrument in which the prothrombin time or the activated partial thromboplastin time of a plasma sample is determined, it is preferred to store the reagent that is to be mixed with the plasma at a temperature of 8.degree. C. and then to raise the temperature of the reagent to 37.degree. C. immediately prior to its being mixed with the plasma. U.S. Pat. No. 3,969,079 discloses such an instrument in which the reagent is cooled and then heated.